Plastic, i.e., polymeric foams, can be generally classified as either closed-cell foams or as open-cell foams. Open-cell foams can be formed, e.g., by emulsion techniques or by phase separation techniques. Open-cell foams can also be made by blowing out the faces of closed-cell foams, e.g., by vacuum and the like. Open-cell foams can be used as a matrix to contain various liquids since the open cells are interconnected.
For example, open-cell foams have been investigated for use as targets in inertial confinement fusion (ICF). Useful polymeric foams for ICF should be rigid, have small pores, a low density, a low atomic number and, of course, be open-celled so they may be filled with the liquid deuterium/tritium (D/T). Additionally, it is desirable to have a foam that will wick or take up the liquid by capillary action. U.S. Pat. No. 4,430,451 discloses low density, microcellular foams of poly(4-methyl-1-pentene) which would be saturated with liquid DT as a fusion target.
Similarly, U.S. Pat. No. 4,806,290 discloses machinable and structurally stable, low density microcellular carbon and catalytically impregnated carbon foams useful for inertial confinement fusion targets.
Other potential beneficial uses for open-cell foams include use as absorbers for toxic and hazardous gases, use in chromatography applications, and use as high surface area catalytic substrates. Each such use of open-cell foams demands a particular combination or range of properties.
Previous microcellular foams have been generally prepared of singular, homogeneous materials, e.g., polyethylene, polypropylene, polystyrene, poly(4-methyl-1-pentene), polyacrylonitrile, carbon, silica aerogel, and formaldehyde-resorcinol. Unfortunately, each of these foam materials is limited in the properties that it can provide. For example, silica aerogel and formaldehyde-resorcinol foams have very fine microstructures, i.e., distances between solid masses of less than 0.5 micrometer, but these foams are extremely brittle and cannot be easily machined or handled. In contrast, while linear organic polymers, such as polystyrene, polyethylene, polymethylpentene, or polyacrylonitrile, can be prepared by phase separation processes to yield foams that are usually machinable, such foams have cellular dimensions generally greater than 5 microns.
There have been previous descriptions of modifying polymeric foams to improve different properties of such foams. For example, U.S. Pat. No. 4,525,386 describes a method of enhancing the properties of a layer of open-cell foam material, such as polyurethane or polyvinylchloride, by impregnating the open-cell foam with filler particles having dimensions in the micron range in order to enhance properties such as mechanical, thermal, electrical or conductive properties.
U.S. Pat. No. 4,454,258 discloses foams made from polyepoxide and polyurethane resins, the foams made with closed-cells which are then crushed to open the cells so that the foam may be impregnated with, e.g., carbon black or other inorganic materials. It is further disclosed that such inorganic fillers can be secured within the cells of the foam material, e.g., by use of a binder material such as a varnish or adhesive, with a phenolic spar varnish being preferred.
U.S. Pat. No. 4,239,571 and 4,230,521 disclose impregnating an initially open-cell foam material with a thermosetting resin, such a resin either alone or in combination with reinforcing fibers. Such a process results in a relatively impervious and rigid composite structure.
Polym. Mater. Sci. Eng. 1988, 58, 1049-1053 entitled "Electrically Conductive Reticulated Carbon Composites" by Sylwester et al discloses another modified polymeric foam involving incorporation of an epoxy resin into a carbon foam. The epoxy resin is cured within the structure of the carbon foam and serves to provide the desired mechanical properties to the conductive carbon foam.
Finally, U.S. Pat. No. 4,832,881 discloses low density, open-celled microcellular carbon foams prepared from acrylonitrile-based materials, such foams useful for fabrication of inertial confinement fusion targets, as catalyst supports, as absorbents, and as filters.
Even with all the previous polymer foams and various modified foams, optimum properties have not always been obtainable. The present inventors undertook a search for foams having tailorable properties such as density, cellular dimensions, compressive strength, capillary action (i.e., wicking of liquids), and formability.
Accordingly, it is one object of this invention to provide a composite polymeric material, e.g., a rigid, machinable composite polymeric foam, having both tailorable densities and microcellular dimensions.
It is a further object of this invention to provide a composite polymeric material, e.g., a composite foam that is structurally stable, machinable, and has a high surface area.
Still a further object of this invention is to provide a cryogenic or inertial confinement fusion target comprising a sphere composed of an open-celled polystyrene or carbon foam, the polystyrene or carbon foam containing a second polymeric foam material within the open cells of the polystyrene or carbon foam, said target having densities and cellular dimensions suitable to hold liquid DT by capillary action.